Frequency multiplexing technique is generally used for increasing system capacity in a cellular communication system. In view of this, inter-cell interference (ICI) becomes a key factor affecting performances of the cellular communication system, especially for cell edge users. Multiple Input Multiple Output (MIMO) technique has become one of key techniques for B3G wireless communication systems including 3GPP Long Term Evolution (LTE) due to its capability of increasing spectrum efficiency of wireless links. However, the performance of MIMO is sensitive to ICI.
As the making of LTE Rel-8 standard drew to a close, the 3GPP started a research on LTE-A in the middle of year 2008. LTE-A is an evolution of LTE and requires better system performances (especially the spectrum efficiency for cell edge users). In order to increase the spectrum efficiency for cell edge users, LTE-A is considering using coordinated transmission technique to cope with the influences of ICI.
Current coordinated transmission technique includes several implementations, three most typical types of which are: fractional frequency reuse (FFR), single-user multi-eNB joint transmission and multi-user multi-eNB joint transmission with full cooperation.
Here, the FFR was described in technical proposals such as 3GPP R1-050507 and 3GPP R1-081873. The FFR may be fixed or dynamic, and it avoids adjacent cells to transmit data simultaneously using the same time-frequency resources in order to prevent cell edge users from being interfered. This scheme is simple in implementation, and does not require eNBs in adjacent cells to share data and Channel State Information at the Transmitter side (CSIT). Nevertheless, since transmission power of eNBs in adjacent cells is not fully utilized, the FFR will lower spectrum efficiency, especially when SNR is low.
The single-user multi-eNB joint transmission was described in technical proposals such as 3GPP R1-083870 and 3GPP R1-084173. According to this scheme, adjacent cells transmit data to a single user simultaneously using the same time-frequency resources in a specific manner, so that the ICI of conventional art can be converted into useful signals. It does not require eNBs from adjacent cells to share CSIT, either. However, this scheme may lead to lowered spectrum efficiency due to inefficient utilization of spatial Degree of Freedom (DoF) brought by multiple antennas of eNBs and users, especially when SNR is relatively high.
The multi-user multi-eNB joint transmission with full cooperation was described in technical proposals such as 3GPP R1-084482 and 3GPP R1-090601. This scheme is optimum in theory, which joins eNBs from multiple cells to form a larger scale transmitting antenna array for serving multiple users using the same time-frequency resources simultaneously. However, this scheme requires eNBs from adjacent cells to submit data and CSIT to a central node for joint processing, which is complex in implementation. Firstly, it will cause large backhaul signaling overhead and long processing delay. Secondly, the central node is highly complex in implementation.